Bya

In astronomy, geology, and paleontology, bya or "b.y.a." is an initialism for billion years ago. This abbreviation is commonly^{[citation needed]} used as a unit of time to denote length of time before the present. Specifically, one bya is equal to 10⁹ years ago.

The "billion" in bya is, by convention, the 10⁹ "billion" of the short scale of the U.S., not the long-scale 10¹² "billion" of some European usage. For this reason, there is potential for some confusion; and some scientists prefer the unit Gya, while others prefer Ga (Giga annum). However, bya remains in more widespread use. In 1974 the U.K. switched from the long scale to the short scale.

Related units are mya ("million years ago"), and byr ("billion years"). These are traditionally written in lowercase. Ga or Gya has a capitalized first letter instead.

See also

• Chronology
• Myr
• annum
• Before Present
• Gigaannum or Ga
• Gya
• mya (unit)
• Orders of magnitude (time)#Years

References